Protection against ReDoS

The regex module is compatible with the re module (VERSION0 flag).
It is also faster.

```python
>>> import re
>>> import regex
>>> import timeit
>>> pattert = "(a+)+b"
>>> input = "a" * 25
>>> timeit.timeit(lambda: re.search(pattern, input), number=10)
32.332445038000515
>>> timeit.timeit(lambda: regex.search(pattern, input, flags=regex.VERSION0), number=10)
0.003861578001306043
>>> input = "a" * 10000
>>> regex.search(pattern, input, flags=regex.VERSION0, timeout=5)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/stsewd/.pyenv/versions/readthedocs.org/lib/python3.6/site-packages/regex/regex.py", line 266, in search
    concurrent, partial, timeout)
TimeoutError: regex timed out
```

readthedocs/builds/models.py

@@ -6,6 +6,7 @@
 import re
 from shutil import rmtree
 
+import regex
 from django.conf import settings
 from django.db import models
 from django.db.models import F
@@ -1137,6 +1138,8 @@ def __str__(self):
 
 class RegexAutomationRule(VersionAutomationRule):
 
+    TIMEOUT = 15  # timout in seconds
+
     allowed_actions = {
         VersionAutomationRule.ACTIVATE_VERSION_ACTION: actions.activate_version,
         VersionAutomationRule.SET_DEFAULT_VERSION_ACTION: actions.set_default_version,
@@ -1146,11 +1149,28 @@ class Meta:
         proxy = True
 
     def match(self, version, match_arg):
+        """
+        Find a match using regex.search.
+
+        .. note::
+
+           We use the regex module with the timeout
+           arg to avoid ReDoS.
+        """
         try:
-            match = re.search(
-                match_arg, version.verbose_name
+            match = regex.search(
+                match_arg,
+                version.verbose_name,
+                # Compatible with the re module
+                flags=regex.VERSION0,
+                timeout=self.TIMEOUT,
             )
             return bool(match), match
+        except TimeoutError:
+            log.warning(
+                'Timeout while parsing regex. pattern=%s, input=%s',
+                match_arg, version.verbose_name,
+            )
         except Exception as e:
             log.info('Error parsing regex: %s', e)
-            return False, None
+        return False, None

requirements/pip.txt

@@ -75,6 +75,7 @@ Unipath==1.1
 django-kombu==0.9.4
 mock==3.0.5
 stripe==2.37.2
+regex==2019.11.1
 
 # unicode-slugify==0.1.5 is not released on PyPI yet
 git+https://github.com/mozilla/unicode-slugify@b696c37#egg=unicode-slugify==0.1.5